Retrofitting of old and historical buildings with respect to their energy consumption has been carried out in various countries mainly by applying a thermal insulation layer either on the external or the internal wall surfaces, depending on the requirements for maintaining style and shape of the façades. For this special application high performance thermal insulation materials such as vacuum insulation panels (VIP) or fiber reinforced aerogel mats have a remarkable asset due to the reduced thickness needed to achieve a required thermal performance. However, these insulations need a plane subsurface, adjustment, gluing and even fastening by means of dowels or have to be produced in precise dimensions.
A different way to insulate buildings is the application of an insulating rendering. A rendering has the advantage of being easily applicable, flexible with respect to surface unevenness and able to fill any gaps in order to provide a continuous thermal insulation layer. So far existing insulation renderings do not reach a thermal conductivity value below 65 mW m−1 K−1, which represents a clear disadvantage compared to the above mentioned high performance insulation materials with a thermal conductivity clearly below 30 mW m−1 K−1. Besides, the typical density of presently used insulation renderings of about 350 kg m−3 does not allow thicknesses beyond 4 to 5 cm without recurring to fixation elements.
In view of the above, it has been proposed in WO 2011/083174 to use an insulating plaster material based on a silica xerogel. This plaster material comprises water, a mineral and/or organic hydraulic binder, optionally a structuralizing filler and additives; moreover it comprises an insulating filler comprising a powder or granules of at least one hydrophobic silica xerogel. In one example, the plaster material produced in a laboratory mixer had a thermal conductivity of about 34 mW m−1 K−1.
Similarly, WO 2011/066209 discloses various thermally insulating composites, including a self-supporting rigid composite that includes an aerogel-containing material and a binder, particularly a cementitious binder. In some examples, low thermal conductivities not exceeding 20 mW m−1 K−1 were achieved. The samples of thermally insulating composite material were prepared by pouring an aqueous slurry in a suitably shaped mold and allowed to dry.
WO 2010/126792 A1 discloses dry blends for producing a composite in the form of a coating. The blends comprise hydrophobic aerogel particles, a surfactant, a binder, fibers and further ingredients. Such possible ingredients comprise silica, perlite, microspheres of glass or polymers and many others.
However, there is still a need for improved thermally insulating rendering or plaster materials. In particular, a rational use of any insulating render unavoidably requires the applicability by means of a plastering machine. In this respect it has been found that the thermal conductivity markedly increases (the insulation performance deteriorates) when samples were produced by a plastering machine as compared to manually produced samples. This performance loss appears to be caused by the overpressure in the plastering machine, as the decrease in thermal insulation was found to be more pronounced with increasing pressure between 0 and 8 bar (relative to atmospheric pressure).
Therefore, it would be desirable to have an improved rendering material having excellent thermally insulating properties, being capable of forming self-supporting elements or layers, and being suitable for application by a plastering machine.